the erlang c traffic model

The Erlang traffic models were established by A.K. Erlang, a
Danish scientist who is credited with much of the early work in telephone
traffic design. He discovered the mathematics underlying queuing, a branch of
statistics now termed 'queuing theory'.

'Erlang' calculators are used
throughout the world to carry out a variety of statistical calculations
associated with telecommunications systems and call centers.

Used by
telephone system designers to estimate the number of lines required for
PSTN connections and takes into account the additional traffic load
caused by blocked callers immediately trying to call again if their
calls are blocked.

Erlang B

Estimates the
number of lines required for PSTN connections (CO trunks) or private
wire connections.

Engset

Used to
explore the relationship between the traffic offered to a trunk group,
the blocking that will be experienced and the number of lines provided
when there are a finite number of sources from which the traffic is
generated. It is more accurate than Erlang B, which tends to
over-estimate blocking.